Anorectal: Anatomy & Physiology

Anatomy

Structures

  • Rectum (10 cm)
    • Rectosigmoid Junction (Debated Location):
      • Sacral Promontory
      • Peritoneal Reflection
      • Level of S3
    • Fascia Mn
      • Denovilliers Fascia (Anterior)
        • Male: Rectovesicular & Rectoprostatic
        • Female: Rectovaginal
      • Waldeyer’s Fascia (Posterior)
        • Rectosacral Fascia
  • Anus (5 cm)
    • Anorectal Ring: Separates Rectum & Anus
      • Corresponds to Levator Ani Muscle
    • Dentate/Pectinate Line
      • Divides Upper 2/3 & Lower 1/3
      • Columnar Epithelium Above
      • Squamous Epithelium Below
    • Anal Verge: Introitus of the Anal Orifice
    • Anal Margin: Perianal Skin, Extends 5-6 cm Out from Anal Verge
    • Intersphincteric Groove: White Line Landmark for Intermuscular Border
    • Anal Canal (2 Different Definitions):
      • Surgical Anal Canal: Anorectal Ring to Anal Margin
      • Anatomic Anal Canal: Dentate Line to Anal Margin
        • Shorter

Sphincters

  • External Anal Sphincter (EAS)
    • Muscle: Puborectalis (Continuation of Levator Ani)
      • Striated Muscle
    • Control: Voluntary
      • Internal Pudendal Nerve (Inferior Rectal Branch)
      • S4 (Perineal Branch)
  • Internal Anal Sphincter (IAS)
    • Muscle: Continuation of Muscularis Propria
      • Smooth Muscle
    • Control: Involuntary
      • Pelvic Splanchnic Nerves

Vascular Supply

  • Rectal Supply
    • Superior Rectal Artery (SRA): From IMA
    • Middle Rectal Artery (MRA): From Internal Iliac Artery
      • Contained in Lateral Stalks in LAR/APR
    • Inferior Rectal Artery (IRA): From Internal Pudendal Artery (From Internal Iliac Artery)
  • Venous Flow
    • Follows Arterial
    • Superior Rectal Vein (SRV) to IMV (Portal System)
    • Middle Rectal Vein (MRV) to Internal Iliac (Caval System)
    • Inferior Rectal Vein (IRV) to Internal Iliac (Caval System)

Lymphatics

  • Rectum
    • Superior Rectum: IMA Nodes
    • Middle Rectum: IMA Nodes
    • Inferior Rectum: IMA (Primarily) & Internal Iliac Nodes
  • Anus
    • Anal Canal: Internal Iliac Nodes
    • Anal Margin: Inguinal Nodes

Anorectal Anatomy 1

Anal Canal 1

Rectal Arteries 2

Mnemonics

Rectal Fascia

  • Into the “Den”: Denovilliers Inward/Anterior
  • Out to the “Wal”: Waldeyer’s to the Posterior Wall/Sacrum

References

  1. Gami B, Kubba F, Ziprin P. Human papilloma virus and squamous cell carcinoma of the anus. Clin Med Insights Oncol. 2014 Sep 17;8:113-9. (License: CC BY-NC-3.0)
  2. Gray H. Anatomy of the Human Body (1918). Public Domain.